Preheat oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners.
Whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt in a large bowl.
Add the egg, milk, oil, and vanilla extract. Beat on medium speed for 2 minutes.
Carefully pour in the boiling water while mixing on low speed until just combined. The batter will be thin.
Divide batter into liners, filling 2/3 full. Bake for 18-22 minutes. Cool completely on a wire rack.
Beat softened butter on medium-high for 5 minutes until pale and creamy.
Gradually add sifted powdered sugar, strawberry powder, and vanilla extract. Mix on low.
Add heavy cream and whip on high for 3 minutes until light and fluffy.
Pipe the buttercream onto the cooled cupcakes and garnish with a fresh strawberry.